Some of the common summer residents are not content with a trip to
northern South America, but push on across the Equator and finally
come to rest for the winter in the pampas of Argentina, or even in
Patagonia. Thus some species that are more or less associated with,
each other in summer, as nighthawks, barn swallows, cliff swallows,
and some of the thrushes may also occupy the same general winter
quarters in Brazil. Some individual nighthawks and barn swallows
(fig. 13) travel still farther, and of all North American land birds
these species probably have the longest migration route, as they
occur north to Yukon and Alaska, and south to Argentina, 7,000 miles
away. Such seasonal flights are exceeded in length, however, by the
journeys of several species of water birds, chiefly members of the
suborder of shore birds. In this group are 19 species that breed
north of the Arctic Circle and winter in South America, 6 of them
going as far south as Patagonia, and thus having a migration route
more than 8,000 miles in length.
[Illustration: B1035M
Figure 14.--Arctic tern. The longest flight known for an individual
bird was accomplished by an arctic tern that in 3 months flew from
the coast of Labrador to the coast of southeastern Africa.]
The arctic tern (_Sterna paradisaea_) is the champion "globe trotter"
and long-distance flier (figs. 14 and 15). Its name "arctic" is well
earned, as its breeding range is circumpolar and it nests as far
north as it can find a suitable place. The first nest to be found
in this region was only 7½° from the North Pole, and it contained
a downy chick surrounded by a wall of newly-fallen snow that had
been scooped out by the parent. In North America it breeds south
in the interior to Great Slave Lake, and on the Atlantic coast to
Massachusetts. After the young are grown, the arctic terns disappear
from their North American breeding grounds, and a few months later
they may be found in the Antarctic region, 11,000 miles away. Until
very recently the route followed by these hardy fliers was a complete
mystery, for although a few scattered individuals have been noted
south as far as Long Island, the species is otherwise practically
unknown along the Atantic coasts of North America and South America.
It is, however, known as a migrant on the west coast of Europe and
Africa. By means of numbered bands the picture is now developing of
what is apparently not only the longest but also one of the most
remarkable of all migratory journeys.
Judging by the evidence at present available, it seems likely that
the arctic terns of eastern North America originally found their way
here from the Old World, probably by way of Iceland and Greenland.
Consequently when the time comes for them to migrate to winter
quarters they do not go directly south as do the common (_Sterna
hirundo_) and Forster's terns (_S. forsteri_), but instead, they fly
back eastward along their ancestral route across the Atlantic to
the shores of Europe and then go south along the African coast to
their winter home; those that breed in the northwestern part of the
continent, as in Alaska, probably migrate chiefly down the western
coast, as the species is not infrequently reported on the coast of
California and also on the western coast of South America.
[Illustration: B4750M
Figure 15.— Distribution and the migration of the arctic terns of
eastern North America. The route indicated for this bird is unique,
as no other species is known to breed abundantly in North America and
to cross the Atlantic Ocean to and from the Old World. The extreme
summer and winter homes are 11,000 miles apart, and as the route
taken is circuitous, these terns probably fly at least 25,000 miles
each year.]
The evidence yielded by banding consists of only three definite
cases, but their interpretation seems to permit but one conclusion:
All three birds were banded as downy chicks, one on July 3, 1913, at
Eastern Egg Rock, Maine,[3] and the other two at the Red Islands,
Turnevik Bay, Labrador, on July 22, 1927, and July 23, 1928. The
first was found dead in the Niger River delta, West Africa, in August
1917, while the Labrador birds were recovered near La Rochelle,
France, on October 1, 1927, and at Margate, near Port Shepstone,
Natal, South Africa, on November 14, 1928. The flight shown by this
last record is the longest known, the trip, between 8,000 and 9,000
miles, being accomplished in less than 3 months.
[Footnote 3: Recorded at the time of banding as a common tern, a
natural error, as the downy young of common and arctic terns look
almost exactly alike.]
Probably no other animal in the world enjoys as many hours of
daylight as does the arctic tern, since for these birds the sun never
sets during their nesting season in the northern part of the range,
while during their sojourn in the south, daylight is continuous.
During several months of the year they have 24 hours of daylight and
during the other months considerably more daylight than darkness.
=ROUTES OF MIGRATION=
While it is beyond question that certain general directions of flight
are constantly followed by migratory birds, it is well to remember
that the term "migration route" is to some extent a theoretical
concept, concerned entirely with the lines of general advance or
retreat of a species, rather than the exact course followed by
individual birds. Even the records of banded birds usually show
no more than the places of banding and recovery, and one must
have recourse to intermediate records and to reasoning based on
probabilities to fill in details of the flyway actually traversed
between the two points.
There is also infinite variety in the routes covered during migration
by different species. In fact, the choice of migration highways is
so wide that it seems as if the routes of no two species coincide.
Differences in distance traveled, in time of starting, in speed of
flight, in geographical position, in latitudes of breeding and of
wintering grounds, and in other factors, all contribute to this
great variation of migration routes. Nevertheless, there are certain
factors that serve to guide the avian travelers along more or less
definite lines, and it is possible to define general lines of
migration for the majority of species.
It has frequently been observed that migrating birds have a tendency
to follow major topographic lines on the earth's surface when their
trend is in the general direction of the birds' journey. Bird
migration is generally thought of as a north-and-south movement,
with the lanes of heavier concentration following the coasts,
mountain ranges, and principal river valleys. To a considerable
extent this is the case, particularly in North America, where the
coast lines, mountain chains, and the larger rivers in general run
north and south. Students of American birds thus have exceptionally
good opportunities to study migratory movements. In cases where the
migration is a long one, however, the notion must be abandoned that
the birds' flight is restricted to particular narrow routes that
follow river valleys and the like, as many species seem to disregard
utterly such apparently good natural flyways as river valleys. For
example, the Arkansas River has a general east and west course for a
great part of its length, and while it does constitute a highway for
many perching birds en route from the Mississippi Valley to the Rocky
Mountain region, some of the hawks and many ducks and shore birds pay
the valley scant attention. They may arrest their fall journey to
feed among cottonwoods or along sand bars, but when ready to resume
their flight they leave the river and fly directly south over the
more or less arid region that lies between the Arkansas and the Rio
Grande.